Abstract: Polished (001) cleavage faces of Nain Labradorite (An almost-equal-to 56) were leached at 20-degrees-C (+/- 2-degrees) for 72 days in distilled water (pH almost-equal-to 5.6), in HCl solution (pH almost-equal-to 4.05) and in four other HCl solutions containing 1 mg/l or less of Na, K, Ca, Al(aq), and SiO2(aq) (pH of all four solutions almost-equal-to 4.05). Analysis of the cleavage faces by Secondary Ion Mass Spectrometry (SIMS) revealed no residual surface layer on faces leached by distilled water. By contrast, thick Si-rich residual layers (1500 angstrom) formed on surfaces leached by pure HCl solutions. Much thinner residual layers (700 angstrom) were observed on labradorite leached by HCl solutions containing 1 mg/l of Na, Ca, and K, whereas leached layers < 75 angstrom thick formed on faces leached by solutions containing 1 mg/l each of Na, K, Ca, Al(aq) and Si(aq); consequently incongruent dissolution of feldspars occurs in HCl solution, but congruent dissolution of feldspars occurs in mixed electrolyte solutions in which dissolved cations are more abundant than H3O+ (hydronium or proton). Of the cations studied, Al(aq) most affects formation of the leached layers, SiO2(aq), however, does not noticeably affect thickness of the residual layer. Affinity of cations for surface functional groups affects the rate of formation of the leached layer (diffusion), and the rate of surface dissolution from the Si-rich surface layer, by controlling the concentration of H3O+ surface complexes on the surface of the leached layer. Diffusion through the leached layer apparently proceeds via lattice diffusion and line defects. A \"competitive adsorption model\" is proposed to explain the SIMS results and experimental leaching results of others. An important prediction of the model is that Si-rich residual layers generally do not form on feldspars of most soil and weathering profiles. If the residues form, they generally will be thin, with the thickness controlled by the composition of the soil solution.